Abstract Although the influence of metabolic rate on longevity has been studied and debated for decades, the cause:effect relationship between different parameters of energy metabolism and the process of aging is poorly understood and controversial. Hypopituitary Ames dwarf (Prop1df) mice and growth hormone receptor deleted (GHRKO) mice are remarkably long-lived and exhibit many features of delayed, slower and healthy aging. Many phenotypic characteristics shared by these mutants are believed to represent mechanisms of extended longevity. These characteristics include increased oxygen consumption (VO2) per gram of total or lean body mass and reduced respiratory quotient (RQ = respiratory exchange ratio, RER), indicative of increased reliance on lipids as metabolic fuel. Intriguingly, the increase in VO2 and the reduction in RQ in Ames dwarf and GHRKO mice disappear or are severely attenuated after 24 hours of acclimation to thermoneutral temperature. From these novel findings, we suspect that increased heat loss and the consequent increase in energy demand for thermogenesis in these diminutive mutants induce alterations in energy metabolism (increased VO2 and fatty acids ? oxidation; reduced RQ) that promote delayed, healthy aging and extended longevity. We hypothesize that chronic exposure of Ames dwarf and GHRKO mice to thermoneutral temperature will accelerate aging and reduce or eliminate their longevity advantage. We further hypothesize that chronic activation of thermogenesis in genetically normal (?wild type?) animals by exposure to reduced environmental temperature will induce metabolic characteristics that are associated with the delayed aging and extended longevity seen in GH-related mutants and thus lead to a slower rate of aging. As the first step in testing these hypotheses, we will determine whether characteristics associated with delayed aging are normalized by extended (weeks to months) exposure of long-lived mutant animals to thermoneutral environment and whether they can be induced by extended exposure of normal animals to reduced environmental temperature. Two Specific Aims are proposed. Specific Aim 1. To determine whether chronic exposure of adult Ames dwarf and GHRKO mice to thermoneutral ambient temperature (30C) will rescue (normalize) energy metabolism (as assessed by measurements of V02, RQ), glucose homeostasis, circulating adiponectin, IL-6 and lipid levels and expression of genes related to mitochondrial uncoupling in brown and white adipose tissue, metabolism, inflammation, insulin and mTOR signaling. Specific Aim 2. To determine whether chronic exposure of adult normal (?wild type?) mice from the same strains to reduced environmental temperature (17C) will induce alterations in thermogenesis, energy metabolism, glucose homeostasis, adipokine levels and expression of selected genes resembling those observed in long-lived GH-related mutants exposed to standard animal room temperature (23C).